Fillers for improved epoxy laminates

ABSTRACT

Epoxy laminates incorporate up to 20 wt. % talc particles, particularly pure Montana platy talc particles having a maximum particle size of about 40 μm providing improved drilling performance, reduced dust formation, and improved Z-direction CTE, particularly when the epoxy resin has a Tg of about 150° C. or higher. The talc is selected from those which do not significantly reduce the electrical strength of the laminate relative to those which contain no talc particles. Characteristically, the talcs will have less than 5 wt. % impurities and less than 0.01 wt. % (100 wt.ppm) water extractable anions.

BACKGROUND OF THE INVENTION

The invention relates generally to the laminates which are widely usedto make printed circuit boards. In one application, the invention isapplied to the composite material used as a substrate for such circuitboards, particularly those made with glass fiber reinforced epoxy resms.

The generally recognized types of laminates are discussed in PrintedCircuits Handbook, Coombs ed., Third Edition, McGraw-Hill Book Co.,1988. The lowest cost materials typically use phenolic resin impregnatedpaper (FR-2) and are used where the cost is more important than theelectrical and physical performance. FR-3 is a paper composite which hasbeen impregnated with epoxy resins rather than phenolic resins. CEM-1 isa composite which is more expensive than the FR-2 and FR-3 materials,but which provides improved electrical and physical properties. ForCEM-1 an epoxy resin is used to coat paper as in FR-3, but the core iscovered with glass fiber reinforced epoxy resins outer layers. FR-4 ismade with a fabric of glass fiber yarns impregnated with epoxy resins.The present invention has application to all types of laminates, but inparticular to FR-4 laminates.

Inorganic fillers are disclosed in U.S. Pat. No. 5,264,065 to be usefulin controlling the coefficient of thermal expansion in the Z-axis oflaminates, generally using 30 to 100 parts of filler per hundred ofresin. Such fillers have been used in laminates for other and relatedpurposes, such as to limit resin flow and to improve punchabilityaccording to the Japanese patent publications (JP 222,950 (1989), JP199,643 (1982) and 7,044 (1984), JP 97,633 (1989), JP 120,330 (1990))discussed in the '065 U.S. patent. In U.S. Pat. No. 4,960,634 zinc oxideis disclosed as an additive for improving thermal conductivity. In JP133,440 (1990) “burned” talc is disclosed to improve dimensionalstability of laminates. The talc was heated to 1000° C.-2000° C. toremove the water of crystallization. Talcs heated to below 1000° C. werestated to lack the desired improved dimensional stability.

The present inventors have been concerned with problems arising from theuse of epoxy resins having relatively high glass transition temperatures(Tg), particularly about 150° C. or higher. These resins have advantagesnot available with those having lower Tg values, but they tend to bemore brittle and more difficult to work mechanically. They produce epoxyresin dust when laminates are punched, cut, or drilled duringfabrication of printed circuit boards. Dust is undesirable for variousreasons, but particularly because it affects the precision with whichcircuit patterns can be made.

The present invention provides a means to significantly reduce theamount of dust produced, with the attendant advantages in manufacturingprinted circuit boards. The invention also provides laminates which maybe more readily drilled. Typically, laminates are stacked and drilled atthe same time for efficiency. If the holes are not drilled cleanly,reworking or rejecting the board may be necessary. Alternatively, thedrilling process may be modified, but this may reduce the speed withwhich the laminates are drilled. An additional advantage of theinvention is that the epoxy resin has lower coefficient of thermalexpansion (CTE) in the Z direction, which reduces circuit failures dueto differential thermal expansion. These advantages are attained byincluding minor quantities of talc in the epoxy resin. However, it hasbeen found that not all talcs can be used for some cause a substantialreduction in electrical strength.

SUMMARY OF THE INVENTION

The invention in one aspect is a method of improving laminates, inparticular those made with epoxy resins having a Tg of 150° C. orhigher, by incorporating in the epoxy resin layers up to about 20 wt. %of talc particles, preferably about 10 to 15 wt. %, which have a maximumparticle size of about 40 μm. The talc should be relatively pure andapproximate the theoretical formula 3 MgO .4 SiO₂.H₂O with less thanabout 0.01 wt. % (100 wt. ppm) water extractable anions. Particularlyuseful are Montana platy talc particles (>96 wt. % talc). They may beeither untreated or have a surface treatment such as a silane. It is animportant characteristic of the talc particles of the invention thatthey do not cause a significant reduction of electrical strengthrelative to laminates which contain no talc.

In another aspect the invention is a laminate for printed circuit boardswhich comprises a reinforcing material in an epoxy resin matrix,particularly those having a Tg of 150° C. or more, and containing up to20 wt. % of talc particles having a maximum particle size of about 40 μmand which do not significantly reduce electrical strength relative tolaminates which contain no talc. A preferred talc is Montana platy talc,which has a relatively high purity (>96 wt. % talc) and approaches thetheoretical composition 3MgO .4 SiO₂ .H₂O with less than about 0.01 wt.% (100 wt.ppm) water extractable anions. Optionally, the talc may besurface-treated, e.g. with a silane.

DESCRIPTION OF THE PREFERRED EMBODMENTS

The addition of solid particles to epoxy resin compositions has beensuggested to lower the cost of laminates and to gain other advantages.Inorganic particles such as clays, talc, etc. typically are lessexpensive than epoxy resins and have been suggested as fillers. Ideally,solid particles could improve toughness of brittle materials bypreventing the propagation of cracks. They have the disadvantage thatthey may be abrasive and cause additional wear to cutting and punchingtools used in the manufacture of circuit boards.

For use in epoxy resins there are a number of requirements which shouldbe met. First, the solid particles must be compatible with thecomponents of epoxy varnishes formulated for application to paper andglass fiber, for example, they should be resistant to aggressivesolvents such as DMF and acetone. The particles should be of a sizewhich permits good dispersion through the varnish and they should not betoo large relative to the distance between the circuit lines. Theyshould not agglomerate and their density should be similar to that ofepoxy resin so that they neither tend to settle nor float on thesurface. It is important that the particles not degrade the electricalproperties of the resin, particularly the electrical strength requiredof laminates. They should not have a significant effect on hightemperature tests which laminates must meet, such as solder float. Theinventors have found that not all solid particles satisfy theserequirements and have discovered certain talcs which provide enhancedperformance when used in laminates, but which do not degrade electricalproperties.

Talc

Talc is a naturally occurring mineral having a theoretical formula 3 MgO.4 SiO₂.H₂O. Since it is a natural material, the composition varies. Oftalcs mined in the U.S., those from Montana are considered to be amongthe purest, (e.g. >96 wt. % talc), while many others contain significantamounts of impurities, which may affect their ability to be used incertain applications. This appears to be the case in the presentapplication, as will be seen in the examples below, where the electricalperformance of laminates can be degraded by less pure talcs.

A major use of talcs is as reinforcing fillers in plastics, where theyprovide improved stiffness and creep resistance. They have beensuggested for use in the laminates which are important components inprinted circuit boards, along with many other types of particulatefillers, as has been noted above. However, the present inventors havefound that all talcs are not equally useful.

After being mined, talc deposits are processed to purify them and toreduce them to fine particles. Commercial products may have a maximumparticle size of about 70 μm, with finer materials having particles nolarger than about 10 μm. Pure talc is very soft and it has a hardness of1 on the Moh scale. The particles have various crystal forms which arecharacteristic of the locations in which they are found. Most of thecommercial talcs have platelike crystals and are termed “platy talcs”.The preferred talcs of the present invention are of this type.

While talcs are considered to be non-conductors of electricity, thepresent inventors have found that not all talcs appear to be equal inthis respect. For use in printed circuit boards certain electricalproperties are important and some talcs have been found to fail therequired electrical breakdown tests, as will be seen in the examplesbelow.

One type of talc which has been suggested for use in electricallaminates in JP 133440 (1990) is termed a “burnt talc” and reference ismade to heating talc at 1,000°-2,000° C. This apparently causes a changein the physical structure of the talc and improves dimensional stabilityof the laminates. The talcs used in the present invention are not ofthis type.

When used in electrical laminates, talc particles should be fine sincethe laminates are thin and the distances between the copper circuitlines can be very small. We prefer that the maximum particle size shouldbe no greater than about 40 μμm. We are not aware of any lower limit onthe maximum particle size, except as it may affect the handling andmixing of the talc particles into the epoxy resin before it is appliedto the reinforcing material.

As will be seen in the examples, it has been found that relatively puretalc which approaches the theoretical formula 3MgO .4 SiO₂.H₂O does notsignificantly reduce the electrical strength as measured by IPC testTM650 2.5.6.2. A laminate made with a less pure talc fails the test. Thereasons for this result have not been completely established, but thepurity of the talc is believed to be an important factor, particularlythe amount of water extractable anions it contains.

A comparison of two talcs was made to determine the differences inimpurities and structure which may correlate to the electrical strengthof the laminates. In an elemental analysis done by XRF (X-RayFluoresence) using a Philips PWI400 Spectrometer an impure talc (Nytal400, R.J. Vanderbilt Company, Inc.) was compared with a pure talc (1731,Whittaker, Clark & Daniels, Inc.), with the following results.

Si % Mg % Ca % Fe % Al % Mn % K % Ti % Cl % S % Na % Cr % Sr % 1731 52.824.7  1.1 3.2 0.6 0.0 0.0 0.1 0.1 0.1 0.0 0.0 0.0 Nytal 400 40.0 24.316.7 0.3 0.1 0.4 0.3 0.1 0.2 0.1 0.2 0.1 0.1

It can be seen that the Nytal 400 contains much more calcium andsignificant amounts of chlorine, sodium, and potassium, which may havecontributed to the loss of electrical strength in laminates made withthis talc.

The samples were also examined by XRD (X-Ray Diffraction) using aPhilips PW3710 Diffractomer. The results showed that the 1731 talc had atalc structure [Mg₃. Si₄O₁₀. (OH)₂] with minor amounts of magnesiumsilicate hydroxide [Mg₃. Si₂O₅. (OH)₄]. However, the Nytal 400 was foundto contain a mixture of talc with calcium magnesium silicate hydroxide[Ca₂. Mg₅. Si₈O₂₂. (OH)₂], magnesium silicate hydroxide [Mg₃. Si₂ 0 ₅.(OH)₄], and a minor amount of sodium calcium magnesium silicatehydroxide.

Further analysis was carried out to determine the water extractableanions in the two talcs, since these could contribute to a loss inelectrical strength. A sample of talc was extracted in pure water at 60°C. for 15 minutes and the water was analyzed using Ion Chromatography(Dionex DX-300). The results were as follows:

Extractable, % Chloride Nitrite Nitrate Phosphate 1731 0.0021 0.000340.00048 0.0034 Nytal 400 0.0031 0.00032 0.00046 0.0284

The higher level of extractable chloride is consistent with thedifference in chlorine content found in the XRF analysis. Solublechlorides have been blamed for loss of electrical properties in otherapplications and consequently, the difference in extractable chloridesfound in the two talcs samples is believed to be a potential cause forloss of electrical strength in laminates containing Nytal 400 talc. Themuch higher level of phosphates may be related to the large calciumcontent of Nytal 400 and they also are believed to contribute to thedeterioration of electrical performance. Consequently, it is believedthat the talcs should contain less than 0.01 wt. % (100 wt.ppm) waterextractable anions.

Use of Talc Particles in Laminates

Conventional electrical laminates are made by impregnating reinforcingmaterials, such as layers of paper, glass fiber mats, or woven glassfiber yarns, with an epoxy resin formulation and then B-staging theimpregnated material. B-staging means that the epoxy resin is partiallycured so that it is not tacky and can be handled and stored for sometime before being used to make a finished laminate. Such materials areoften called “prepregs”. When assembled with copper foils and otherlayers of prepreg or fully cured composites, the prepregs can be curedunder heat and pressure to complete the curing process (C-staged).

During the fabrication of printed circuit boards, there are manyoccasions when a laminate is cut, drilled, and punched, all of whichoperations create epoxy resin dust. Such dust is undesirable for manyreasons. It will be appreciated that manufacturing of dense electroniccircuits will be adversely affected by dust which accumulates on theboard and which may affect the electrical continuity of the circuits orinterfere with their construction. Drilling can also be a problem,particularly when resins are used which have a glass transitiontemperature (Tg) above that of the typical FR-4 laminate (about 130°C.). Such resins tend to be more brittle and difficult to drill cleanly.It becomes necessary to reduce the “chipload”, i.e., the length of thehole drilled for each revolution of the drill bit, and consequentlyproductivity is reduced. The inventors have sought and have found amethod for improving the manufacturing process and reducing dusting ofhigher Tg epoxy resins, while at the same time reducing the problemsassociated with drilling.

Adding talc particles has been found to provide the needed improvementto the epoxy laminates, when the amount added is up to about 20 wt % ofthe laminate based on the resin content. Greater amounts could affectthe physical properties adversely. Preferably, the amount used should bebetween about 10 and 15 parts per hundred (phr) based on the resincontent. The maximum particle size should be about 40 μm. Largerparticles will be difficult to distribute uniformly throughout the epoxyresin and would be expected to weaken the structure if too large. Thereis no minimum acceptable size of the particles known to the presentinventors.

Epoxy resin formulations comprise the resins themselves plus curingagents, chain extenders, catalysts, and additives as required. Theinvention is directed particularly to compositions which when cured havea glass transition temperature (Tg) of about 150° C. or higher whichtend to be more brittle than the epoxy resins used in common FR-4laminates and, consequently, more difficult to drill and prone to dustformation during processing of printed circuit boards. However, theinvention may have application to epoxy resins having a lower Tg or toother polymers where difficulty is experienced with drilling or punchinglaminates owing to brittleness.

The talc particles may be added to the epoxy resin formulation by anyconvenient method. However, since the formulation will usually besomewhat viscous, obtaining a uniform distribution of the particles mayrequire high shear mixing to avoid agglomeration of the particles.

In the examples which follow an epoxy resin formulation was applied to afabric made of glass yarn and then fully cured to make laminates. Theselaminates were subjected to tests for drillability and dusting and theZ-axis thermal expansion. The electrical strength was found to beaffected by the addition of talc to the epoxy resins and it wasdiscovered that the purity of the talc was important if electricalstrength of the laminate was to be maintained.

EXAMPLE 1 (Comparative)

A resin formulation was made without the addition of talc.

XUS 19041 ⁽¹⁾ 331.5 g  PM Glycol Ether 48.0 g  2MI (2-methyl imidazole)2.1 g Dicyandiamide 2.1 g Durite SD-357B ⁽²⁾ 2.1 g ⁽¹⁾ epoxy resin (DowChemical), which has a cured Tg of about 150° C. ⁽²⁾ tetraphenol ethane(Borden Chemical)

Laminates having a thickness of 0.059″ (1.5 mm) were made from the aboveformulation using a glass yarn fabric (7628 Clark-Schwebel). The coatedfabric was cured at 171° C. for 3 minutes to produce a prepreg. Then 1oz/ft²(35 μm thick) copper foil was applied to each face of the prepregand the laminate cured at 177° C. for 70 minutes. To determine thedrillability of the laminates, standard drilling parameters used forless brittle 130° C. Tg laminates were used and four laminates weredrilled in a stack. The above laminate drilled burr-free on only 1 outof 3 drilling trials, indicating that the laminates made with a nominal150° C. Tg epoxy resin could not be drilled satisfactorily using drillpractices found acceptable for 130° C. Tg epoxy resins.

Laminates having a thickness of 0.008″ (0.2 mm) were made from the aboveformulation and when tested under IPC test TM650 2.5.6.2 were found toexhibit Electrical Strength exceeding 1250 volts/mil (49212 volts/mm)when measured under D48/50 conditioning (48 hrs. immersed in water at50° C.). The Military Specification for minimum performance is 750volts/mil (29528 volts/mm).

It was concluded that the above laminates did not drill well, but hadgood electrical performance.

EXAMPLE 2

To the formulation of Example 1 was added a relatively less pure talc,Nytal 400 (a New York talc from R.T. Vanderbilt Company), which containsabout 7 wt. % CaO and has a maximum particle size of 20 μm.

XUS 19041 Resin 331.5 g  PM Glycol Ether 54.0 g  2MI (2-methylimidazole) 2.1 g Dicyandiamide 2.1 g Durite SD-357B 2.1 g Nytal 400 30.0g 

Again, 0.059″ (1.5 mm) laminates were made from the above formulationand were drilled in stacks. As before, the standard drilling parametersused for 130° C. Tg laminates were used and the laminates were drilledwhen stacked four high.

The above laminate drilled burr-free on all 3 drilling trials,indicating that the addition of the talc improved the drillingproperties of the 150° C. Tg epoxy resin.

When 0.008″ (0.2 mm) laminates made from the above formulation weretested, they were found to exhibit Electrical Strength between 600 to700 volts/mil (23622-27559 volts/mm) in the IPC test of Example 1, whichis below the Military Specification for minimum performance of 750volts/mil (29528 volts/mm).

It was concluded that the addition of the impure talc improved drillingof the above laminates, but produced unacceptable electricalperformance.

EXAMPLE 3 (Comparative)

To the control formulation of Example 1 was added an epoxidized rubberfor comparison.

XUS 19041 Resin 331.5 g  PM Glycol Ether 48.0 g  2MI (2-methylimidazole) 2.1 g Dicyandiamide 2.1 g Durite SD-357B 2.1 g Epon 58005 ⁽¹⁾7.5 g ⁽¹⁾ epoxidized butadiene-acrylonitrile polymer (Shell ChemicalCo.)

The 0.059″ (1.5 mm) laminates made from the above formulation weredrilled three times as before, using standard drilling parameters for130° C. Tg laminates.

The laminates drilled burr-free on 2 out of 3 drilling trials,indicating that substitution of the epoxidized rubber had a beneficialeffect on drilling performance, although it is considered inferior tothe use of talc. In addition, the talc is much less expensive and wouldbe preferred for that reason.

EXAMPLE 4

To the control formulation of Example 1 was added a pure Montana platytalc for comparison with the less pure talc of Example 2. This talccontains only about 0.1-0.3 wt. % CaO.

XUS 19041 Resin 350.0 lbs. PM Glycol Ether 70.0 lbs. 2MI (2-methylimidazole) 2.217 lbs. Dicyandiamide 2.217 lbs. Durite SD-357B 3.801 lbs.1731⁽¹⁾ 31.7 lbs. ⁽¹⁾Whittaker, Clark & Daniels, Inc. (maximum particlesize 40 μm)

Again, 0.059″ (1.2 mm) laminates made from the above formulation weredrilled three times using standard drilling parameters for 130° C. Tglaminates were used.

The above laminates drilled burr-free on all 3 drilling trials,indicating superior performance of this talc.

When 0.008″ (0.2 mm) laminates made from the above formulation weretested, they exhibited Electrical Strength exceeding 1300 volts/mil(51,181 volts/mm) in the IPC test of Example 1, indicating that theelectrical strength had not been reduced from that of Example 1.

Thus, the beneficiated Montana platy talc, of which 1731 is an example,results in improved drilling performance as well as good electricalperformance.

EXAMPLE 5

A series of laminate samples were made using a formula correspondinggenerally to those of the previous examples, but differing in the talcsincluded, in order to determine the relationship between the extractableanion content and the electrical strength of the laminates.

XUS 19041 Resin 75.31 wt % PM Glycol Ether 16.12 wt % 2MI (2-methylimidazole) 0.48 wt % Dicyandiamide 0.48 wt % Durite SD-357B 0.8 wt %Talc 6.81 wt %

Using Nytal 400 and 1731 alone and in mixtures the electrical strengthwas measured in the each laminate, which are reported in the tablebelow, along with the calculated water extractable anions.

% Water Talc volts/mil (volts/mm) Extractable Anions 100% Nytal 400 625(24,812 volts/mm) 0.03228 75% Nytal 400/ 678 (26,916 volts/mm) 0.0257825% 1731 50% Nytal 400/ 720 (28,584 volts/mm) 0.01930 50% 1731 25% Nytal400/ 904 (35,889 volts/mm) 0.01281 75% 1731 100% 1731 1348 (53,515volts/mm)  0.00632 100% Talkum H-CH15⁽¹⁾ 1382 (54,865 volts/mm)  0.00803⁽¹⁾Scheruhn Industrieminerolien GmbH & Co.

It is evident that the electrical strength measurements correlate wellwith the extractable anions. Another talc which has low extractableanions and which does not reduce the electrical strength is included toconfirm the correlation.

With the addition of finely powdered talc to the 150° C. Tg resinformulation at approximately 10 phr levels, quite unexpectedly, we haveobserved a reduction in the dusting of the prepreg as measured by the“Dusting Test.” The “Dusting” test measures the amount of dust loss thatoccurs when the prepreg is sheared a certain number of times. Morespecifically, panels 5″×5″ (127 mm ×127 mm) were weighed and then werecut into 0.5″ (12.7 mm) wide strips using a paper cutter. The cut edgeswere brushed to remove loose particles and weighed again. The differencein weight was designated the “dust” produced by cutting the panels.

EXAMPLE 6 (Comparative)

A formulation without talc was made into laminates and tested for theamount of dust made in the test described above.

XUS 19041 Resin 331.5 g  PM Glycol Ether 48.0 g  2MI (2-methylimidazole) 2.1 g Dicyandiamide 2.1 g Durite SD-357B 2.1 g

The above control formulation was found to exhibit a dust loss between2.5 to 3.0% as measured via the “Dusting” test.

EXAMPLE 7

A similar formulation was made using the relatively less pure talc ofExample 2.

XUS 19041 Resin 364.7 lbs. PM Glycol Ether 59.4 lbs. 2MI (2-methylimidazole) 2.31 lbs. Dicyandiamide 2.31 lbs. Durite SD-357B 2.31 lbs.Nytal 400 33.0 lbs.

Laminates made from the above formulation were found to exhibit a dustloss of 1.17% as measured via the “Dusting” test.

EXAMPLE 8

The purer talc of Example 4 was used in the formulation and laminatesmade and tested as before.

XUS 19041 Resin 350.0 lbs. PM Glycol Ether 70.0 lbs. 2MI (2-methylimidazole) 2.217 lbs. Dicyandiamide 2.217 lbs. Durite SD-357B 3.801 lbs.1731 31.7 lbs.

The laminates made with the above formulation were found to exhibit adust loss of 1.28% as measured via the “Dusting” test.

EXAMPLE 9

The formulation of Example 8 was repeated, except that a surface-treatedMontana platy talc was used.

XUS 19041 Resin 350.0 lbs. PM Glycol Ether 70.0 lbs. 2MI (2-methylimidazole) 2.217 lbs. Dicyandiamide 2.217 lbs. Durite SD-357B 3.801 lbs.Polytalc 262⁽¹⁾ 31.7 lbs. ⁽¹⁾Whittaker, Clark, & Daniels, Inc. (maximumparticle size 30 μm)

The laminates made from the above formulation were found to exhibit adust loss of 1.69% as measured via the “Dusting” test.

EXAMPLE 10

The formulation of Examples 8 and 9 was made with the substitution of asilane-treated Montana platy talc.

XUS 19041 Resin 350.0 lbs. PM Glycol Ether 70.0 lbs. 2MI (2-methylimidazole) 2.217 lbs. Dicyandiamide 2.217 lbs. Durite SD-357B 3.801 lbs.Vertal 710 ⁽¹⁾ 31.7 lbs. ⁽¹⁾ Luzenac America Inc. (maximum particle size50 μm)

Laminates made with the above formulation were found to exhibit a dustloss of 1.61% as measured via the “Dusting” test.

As shown in the above Examples 6-10, finely powdered talc when used as afiller results in lowering the dust loss during shearing of prepreg. Thereduction in dust loss appears to be independent of the talc used.

With the addition of finely powdered talc to the 150° C. Tg resinformulation at approximately 10 phr levels, we have observed asignificant reduction in the Z-direction coefficient of thermalexpansion (CTE) of 0.059″ (1.2 mm) rigid laminates over the controlformulation that does not contain talc. A reduction in the Z-directionCTE is a highly desirable attribute as it results in improvedreliability of plated through holes in circuit boards. A lowerZ-direction CTE implies a lower mis-match in CTE between the laminateand copper. This results in less differential expansion and less stressbuild-up between the copper/laminate interface in the plated throughhole.

EXAMPLE 11

XUS 19041 Resin 331.5 g  PM Glycol Ether 48.0 g  2MI (2-methylimidazole) 2.1 g Dicyandiamide 2.1 g Durite SD-357B 2.1 g

When 0.059″ (1.2 mm) laminates made from the above formulation weremeasured for Z-axis thermal expansion using the Thermo-MechanicalAnalyzer (TMA). The data obtained as follows:

CTE (ppm/° C.) < Tg  65 > Tg 290 20° to 288° 190

EXAMPLE 12

The pure Montana platy talc of Example 4 and 7 was added.

XUS 19041 Resin 1326 lbs. PM Glycol Ether 264 lbs. 2MI (2-methylimidazole) 8.4 lbs. Dicyandiamide 8.4 lbs. Durite SD-357B 8.4 lbs. 1731120.0 lbs.

The 0.059″ (1.2 mm) laminates made from the above formulation weretreated as in Example 10. The Z-direction thermal expansion measuredusing the Thermo-Mechanical Analyzer (TMA) was as follows:

CTE (ppm/° C.) < Tg  53 > Tg 254 20° to 288° 166

It is evident from examples 10-12 that with the addition of 1731 talcthere is a significant reduction in the net Z-direction (out -of-plane)coefficient of thermal expansion.

What is claimed is:
 1. A method of reducing dusting in epoxy resin basedlaminates during processing of said laminates comprising incorporatingin said epoxy resin more than 0 and up to about 20 wt % of talcparticles wherein the talc particles have less than about 0.01 wt %water extractable anions and wherein the talc particles are not burnttalc particles.
 2. The method of claim 1 wherein said epoxy resin basedlaminate has a Tg of about 1500° C. or more.
 3. The method of claim 1wherein said talc particles have the formula 3MgO. 4SiO2. H₂O with lessthan about 5 wt. % impurities.
 4. The method of claim 3 wherein saidtalc particles consist of Montana platy talc.
 5. The method of claim 1wherein said talc particles have a maximum particle size of 40 μm. 6.The method of claim 1 wherein said talc particles are surface-treated.7. The method of claim 1 wherein said talc particles are surface-treatedwith a silane.
 8. The method of claim 1 wherein said laminate containsabout 10 to 15 wt. % of said talc particles based on the resin content.